HENRY MOLAISON ANDI sat opposite each other, a microphone on the narrow table between us.

"I understand that you have a little trouble remembering things," I said.

"Yes, I do. I do have — well, a lot of trouble remembering things, you know."

"How long have you had trouble remembering things?"

"That, I don't know myself. I can't tell you because I don't remember."

"Well, do you think it's days or weeks? Months? Years?"

"Well, see, I can't put it in exactly on a day, week, or month, or year basis."

"But do you think it's been more than a year that you've had this problem?"

"I think it's about that. One year or more. Because I believe I had an — this is just a thought that I'm having myself — that, well, I possibly have had an operation or something."

Our conversation took place in May 1992, nearly 40 years after Henry's capacity to form long-term memories disappeared as a result of a risky surgical intervention. In 1953, when he was 27, he underwent a bilateral medial temporal-lobe resection, an experimental brain operation intended to alleviate the severe epilepsy he had faced since childhood. The operation did control his seizures, but with an unanticipated and devastating consequence — an extreme amnesia that robbed Henry of the ability to form new memories and, in doing so, determined the course of the rest of his life.

From the beginning, one of the most striking aspects of Henry's memory loss was how remarkably specific it was. He forgot all of his experiences after his operation, but retained much of what he had learned before that. He knew his parents and other relatives, recalled historical facts he had learned in school, had a good vocabulary, and could perform routine daily tasks, such as brushing his teeth, shaving, and eating. One important lesson scientists have learned from people with selective memory loss such as Henry's is that memory is not a single process but a collection of many different processes.

HENRY'S CASE SHED light on a long-standing controversy about whether brief memory mechanisms are distinct from lasting ones. The basic question was whether the processes that support short-term memory, which holds a limited amount of information temporarily, differ from those that support long-term memory, which hangs on to vast amounts of information for minutes, days, months, or years.

Short-term memory, as defined by memory researchers, does not refer to recalling what we did yesterday, this morning, or even 20 minutes ago. That sort of recollection is recent, long-term memory. Short-term memory is the immediate present, the information on our radar screens at this very moment; it expires within about 30 seconds, depending on the task. Its capacity is limited, and it fades immediately if we do not rehearse it or convert it into a form that can be retained in long-term memory. When I tell a friend my phone number, the digits will remain in her short-term store briefly, and she will quickly forget them unless she mentally processes them or writes them down. Long-term memory is anything we remember after just seconds have elapsed.

Was the formation of short- and long-term memory part of a single process, or instead, governed by wholly separate processes? Those who supported the dual-process theory sought evidence that a particular patient was impaired on tests of long-term but not short-term memory, and that another patient was impaired on short-term memory tasks but not long-term. These two results, taken together, would indicate that the two kinds of memory were independent. Henry played a starring part in this research.

If the single-process theory were correct, then Henry's short-term memory should have been compromised. As it was, his short-term memory remained intact, while his long-term memory disappeared, suggesting that not only were they separate processes, but they also depended on different areas of the brain.

Psychologist Brenda Milner saw that Henry could provide experimental evidence to address the single-process versus dual-process debate. In 1962 Milner's graduate student Lilli Prisko gathered data on his short-term memory. She asked Henry to compare two simple, nonverbal stimuli that were separated by a brief delay; the challenge for him was to hold the first item in his memory long enough to say whether it was the same or different from the second item. Prisko chose several different kinds of test items, enabling her to collect data from Henry in multiple experiments. Some used sounds such as clicks and tones, and others used visual images such as light flashes, colors, and non-geometric nonsense figures.

In the first experiment, the stimulus was a series of flashes from a strobe light at a rate of three per second. After a brief delay, another set of flashes appeared, moving faster now at about eight per second. Henry had to say "Different" to indicate that there had been a change between the two stimuli. On other trials, the flashes appeared at the same rate, and he had to say "Same."

The results of this experiment provided an important basis for understanding Henry's capacity for perceiving and retaining information. He could easily and accurately perform the task when there was no delay between stimuli, making only one error in 12 trials. He clearly had no problem with the instructions or perceiving the test stimuli.

Prisko next tested Henry with the same sets of flashes, this time separated by 15, 30, or 60 seconds. The ability to differentiate between stimuli becomes harder for everyone as the gap between them gets longer and as short-term memory weakens. In Henry's case, however, the differential was extreme. With a 15-second interval, Henry still performed well, making two errors out of 12 trials; when the delay was 30 seconds, his errors increased to four. At 60 seconds, his answers were wrong on six of the 12 trials, no better than if he had flipped a coin each time to choose his answer. In comparison, normal control subjects made an average of one error out of 12 during the 60-second delay between stimuli.

The abrupt breakdown in Henry's performance showed that his short-term memory lasted less than 60 seconds. Somewhere between 30 and 60 seconds, his memory of what he had seen or heard crept away.

BECAUSE HENRY COULDrely only on short-term memory, he harnessed cognitive control processes to compensate. By mentally rehearsing information he was asked to remember, he could sometimes keep thoughts fresh in his mind until asked to retrieve them. Milner noticed this ability during her first testing session with Henry in 1955. She gave Henry these instructions: "I want you to remember the numbers five, eight, four." She then left the office. Twenty minutes later, she returned and asked Henry, "What were the numbers?"

"Five, eight, four," he replied.

"Oh, that's very good!" she said. "How did you do that?"

"Well, five, eight, and four add up to 17," Henry answered. "Divide by two, you have nine and eight. Remember eight. Then five — you're left with five and four — five, eight, four. It's simple."

"Well, that's very good. And do you remember my name?"

"No, I'm sorry. My trouble is my memory."

"I'm Dr. Milner, and I come from Montreal."

"Oh, Montreal, Canada," Henry said. "I was in Canada once — I went to Toronto."

"Oh. Do you still remember the number?"

"Number? Was there a number?" The complex calculations Henry had devised to keep the number in his head were gone. As soon as his attention was diverted to another topic, the content was lost.

OUR UNDERSTANDING OF short-term memory has become more complex as we study how we use our memories in everyday life. As we take in information from the world, we engage numerous complex processes in the brain. If someone tries to multiply 68 by 73 in her head, she is performing calculations, saving the results, combining numbers, and checking their accuracy. The task requires much more effort than just regurgitating items held in her short-term memory; it is mental labor. This sort of process is called working memory: an effortful extension of short-term memory, a mental workspace where cognitive operations take place.

Current models of working memory emphasize the interactions between short- and long-term memory. Cognitive neuroscientists have championed the view that working memory integrates specific information from different time periods — the sights, sounds, smells, tastes, and skin and body sensations that just entered the brain, and the contents of long-term memory that are relevant to these inputs.

Imagine a woman in a restaurant who is listening to a waiter list the day's specials. She keeps the list of dishes active in her working memory while simultaneously evaluating each dish, based on knowledge stored in her long-term memory. After shuffling these options in her head, she decides not swordfish, because of its mercury content; not fried chicken, because it has too much fat; but the vegetarian pasta sounds similar to a dish I liked before. She orders the pasta. Although she made this decision quickly, it came about through cooperation across networks of brain areas, enabling her to monitor and manipulate different kinds of information.

Henry's working memory was sufficiently robust to allow him to play bingo, speak in sentences, and do simple arithmetic in his head. He was unable, however, to integrate his online thoughts with memories of the recent past. If he ordered a meal in a restaurant, he could make a choice based on what he liked and disliked before his operation, but he could not take into account what he had eaten the day before, whether he had to select low-calorie items to control his weight, or if he needed to limit his salt intake. Henry depended on his caregivers to fill in that information and much more. His daily life had many limitations because he lacked vital long-term memory capacities.

No one would doubt that Henry's experience was a tragedy, but he rarely seemed to suffer and was not continuously lost and frightened — quite the contrary. He always lived in the moment, fully accepting the events of daily life. From the time of his operation, every new person he met was forever a stranger, yet he approached each one with openness and trust.

Henry was free from the moorings that keep us anchored in time, attachments that can sometimes be burdensome. Our long-term memory is critical to our survival, but it also hinders us; it prevents us from escaping embarrassing moments we have lived though, the pain we feel when thinking about lost loved ones, and our failures, traumas, and problems.

When we consider how much of the anxiety and pain of daily life stems from attending to our long-term memories and worrying about and planning for the future, we can appreciate why Henry lived much of his life with relatively little stress. He was unencumbered by recollections from the past and speculations about the future. As frightening as it seems to live without long-term memory, a part of us all can understand how liberating it might be to always experience life as it is right now, in the simplicity of a world bounded by 30 seconds.